1. Field of the Invention
The present invention relates to a boiler tube and, more particularly, to a boiler tube having a double tube structure.
2. Description of the Prior Art
Hitherto, in the boilers in which the steam temperature in the tube is not higher than 570.degree. C. and the temperature of the tube outer wall surface contacting the combustion gas is between 600.degree. and 650.degree. C., low-alloy steels such as 2.multidot.1/4 Cr-1 Mo steel and 9 Cr-1 Mo steel are generally used as the material of the tubes disposed in the region where the temperature is comparatively low, while austenitic stainless steels such as SUS 304, SUS 321 and SUS 347 are used as the materials of the tubes disposed in the region where the temperature is comparatively high.
In recent years, to cope with the demand for higher efficiency of electric power generation and so forth, there is a tendency to operate the boilers at high steam temperature of 600.degree. C. and yet higher temperature of 650.degree. C., and at a correspondingly increased steam pressure. This requires a 30.degree. to 100.degree. C. rise of the steam temperature in the boiler tubes and a corresponding rise of the tube outer wall surface temperature. Although the austenitic stainless steels are satisfactorily used as the material of the boiler tubes in the region of comparatively high temperature of the boiler when the steam temperature is around 570.degree. C., steam oxidation of the boiler tubes made of austenitic stainless steel is drastically increased and the corrosion of the outer wall surfaces of tubes by the combustion gas is accelerated when the steam temperature is raised to 600.degree. C.
Under these circumstances, boiler tubes having double tube structure, consisting of an inner tube having a high resistance to steam oxidation and an outer tube having a high resistance to high-temperature corrosion, have been proposed as boiler tubes specifically intended for use at high temperatures. An example of boiler tubes having double-tube structure is shown in Japanese Patent Application Laid-Open No. 57-120002.
In recent years, there is a trend for higher steam temperature also in coal burning boilers which makes use of coal as the fuel. The mechanism of high-temperature corrosion caused by the coal combustion gas is different from that caused by the heavy oil combustion gas. Therefore, the material highly resistant to the high-temperature corrosion caused by the heavy oil combustion gas does not always exhibit high resistance to the high-temperature corrosion caused by the coal combustion gas. Thus, there is an increasing demand for development of boiler tubes which simultaneously meet the requirements for high resistance to steam oxidation, excellent high-temperature strength and high resistance to high-temperature corrosion.